Hydrostatic drive

ABSTRACT

The invention relates to a hydrostatic drive as a whole system, comprising two fluidic displacement units ( 1, 3 ), which can be adjusted at least in respect of the volumetric flow and of which one is coupled to an input ( 9 ) and the other is coupled to an output ( 11 ) and which can be connected to each other in the manner of a closed fluidic circuit, to which a storage circuit ( 23 ) is connected, which has at least one storage device ( 33 ) and which is divided into a low-pressure side ( 27 ) and a high-pressure side ( 25 ), and comprising a valve control device ( 17 ) for controlling the whole system.

The invention relates to a hydrostatic drive as an overall system, comprising two fluidic displacement units, which can be adjusted at least in respect of the volumetric flow, and of which one is coupled to a power unit and the other is coupled to an output drive and which can be connected to each other in the manner of a closed fluidic circuit.

Drives of this kind (DE 10 2009 058 005 A1) are known per se and are commonly used as traction drives in commercial vehicles. In an application of this kind it is common that one of the displacement units is driven directly by a combustion engine, and the other displacement unit at the output drive side is coupled with the respective drivetrain.

Based upon the above-described prior art it is the object of the invention to provide a hydrostatic drive that is characterized by a particularly cost-effective and energy-efficient operating performance.

According to claim 1 this object is met in a hydrostatic drive of the kind described at the outset in that an accumulator circuit is provided to which a fluidic circuit is attached that connects the displacement units, wherein the accumulator circuit is provided with at least one accumulator unit and where the accumulator circuit is subdivided into a low-pressure and a high-pressure side, and wherein a valve control device is provided to control the overall system. With a system design of this kind it is possible to realise, over and above the pure drive function, not only brake energy recovery but further special functions such as overspeed protection during brake energy recovery in overrun mode or hydraulic boosting of the drive performance.

Advantageously the system may be designed such that, when the fluidic circuit is closed, both displacement units are connected to each other on their respective input and output side each with a load line, wherein the valve control device is disposed essentially between the two load lines.

In this instance the valve control device may be provided with two first valve devices which, in their open position, connect the two displacement units via the respective load line, and which in their shut-off position disconnect at least the high-pressure side from the low-pressure side of the overall system, wherein the displacement unit of the output drive side is assigned to the low-pressure side and the displacement unit of the power unit side is assigned to the high-pressure side of the overall system.

In particularly advantageous exemplary embodiments a second valve device each, with non-return function, is placed in the circuit between the two load lines and the low-pressure side accumulator circuit. As a result of the non-return function of this valve device, the pressure medium, which is at minimum pressure, is able to spread through the overall system and thus ensures a close to constant low pressure level in the overall system, even with the traction drive in operation.

In a particularly advantageous manner it is possible to place a further third valve device with proportional function between the two load lines and the high-pressure side accumulator circuit. This provides the possibility of a proportional adjustment of the desired working pressure in the respective load line, but also serves as non-return valve to charge the accumulator device when not in operation, and for Start-Stop operation.

In particularly advantageous exemplary embodiments a permanent fluid supply is connected to the low-pressure accumulator circuit between the second valve devices and the output drive side displacement unit. A constant pressure supply of this kind ensures that the low-pressure side is retained at the system-specific charging pressure level. The overall system is also filled via this supply line, and any volume losses that may occur are also replenished, for example if due to pressure spikes fluid is released via a pressure relief valve and flows back to the tank, or losses occur due to leakage.

A particularly advantageous arrangement may be that the permanent fluid volume supply is connected to a feed line at its inlet and which vents into a tank at the outlet side, wherein the feed line is permanently connected to the low-pressure accumulator circuit via a connection point, and where a non-return valve is disposed into the feed line between the connection point and the inlet, and where a pressure relief valve is disposed in said feed line between the connection point and the outlet.

Moreover, in an advantageous manner two further, fourth valve devices with proportional pressure relief function may be disposed between the two load lines, in the line section between the two first valve devices as well as the two third valve devices, extending parallel to the latter.

Between the pairs of third and fourth valve devices with their associated connecting lines, a further connecting line may advantageously be placed, into which a further non-return valve is inserted.

In a particularly advantageous manner the accumulator device of the accumulator circuit may essentially consist of a double-piston accumulator, the double-piston of which is guided longitudinally moveable in an accumulator housing, which separates a first working chamber, applied with a charging pressure, from a second working chamber on the high-pressure side as well as from a third working chamber on the low-pressure side and a fourth working chamber at atmospheric pressure from each other. When used for traction drives of mobile units, which usually provide a limited amount of installation space for the hydraulic system, the design according to the invention with a double-piston accumulator, which performs the function of two hydraulic accumulators, thus ensures a particularly compact design of the system, which is of particularly significant advantage. Moreover, this results in a low-pressure side with additional compensation accumulator.

The respective displacement unit may advantageously consist of a four-quadrant system with adjustable pivoting angle, which may be operated as a hydraulic motor as well as a hydraulic pump.

According to claim 13 an object of the invention is also a valve control device as a subsystem, in particular for an overall system in form of a hydrostatic drive according to one of the claims 1 to 12.

The invention is now explained in greater detail by way of an exemplary embodiment depicted in the drawing.

Shown are in:

FIG. 1 a system diagram of the overall system of an exemplary embodiment of the drive according to the invention, designed as a traction drive;

FIG. 2 a symbolic diagram of the circuit of the exemplary embodiment, wherein for the operating state “Charge Accumulator”, when at standstill, high-pressure fluid paths are shown in full lines, low-pressure fluid paths are shown in broken lines and the flow direction is indicated by arrows;

FIG. 3 a diagram that corresponds to that of FIG. 2, wherein the operating state “Charge Accumulator” is shown in drive mode;

FIG. 4 a corresponding diagram that depicts the operating state “Brake Energy Recovery/Overspeed Protection”;

FIG. 5 a corresponding diagram that depicts the operating state “Start Combustion Engine”;

FIG. 6 a corresponding diagram that depicts the operating state “Boost”.

The diagram in FIG. 1 depicts on the power unit side a displacement unit with the reference number 1 and on the output drive side a displacement unit with the reference number 3. Each displacement unit 1 and 3 comprises a pump motor unit 5 and 7 respectively in form a of a four-quadrant system with adjustable pivoting angle. On the displacement unit 1 on the power unit side the pump motor unit 5 is coupled via a drive shaft 9 with a combustion engine or an electric motor (not shown). At the displacement unit 3 on the output drive side, the pump motor unit 7 is coupled via an output shaft 11 to the drive train (gearbox) of a vehicle (not shown). One connection of the displacement units 1 and 3 is connected to a first load line 13, and the other connection of each of the displacement units 1, 3 is connected to a second load line 15. A valve control device is referenced as an overall unit with the number 17 and is assigned to said load lines 13 and 15. The two connections of the pump motor units 5 and 7 of the displacement units 1 and 3 are each connected to one of the input ports of a shuttle valve 19, the output port of which is connected via a pressure relief valve 21 to tank T. Thus, the connection of the pump motor units 5 and 7, which carry the higher pressure level, and with it the connected load lines 13 and 15, are protected via the respective pressure relief valve 21 towards the tank against system overpressure.

An accumulator circuit 23 with a high-pressure side 25 and a low-pressure side 27 is assigned to the valve control device 17. The high-pressure side 25 is connected to the valve control device 17 at a connection point 29, and the low-pressure side 27 is connected to the valve control device 17 at a connection point 31. The accumulator device of the accumulator circuit 23 is a hydro-pneumatic double-piston accumulator 33. Its double-piston 35, which is guided longitudinally moveable in accumulator housing 37, separates in accumulator housing 37 a first working chamber 39 that contains a process gas, in particular N₂, under charging pressure, from a second working chamber 41, which is connected to the low-pressure side 27 of the accumulator circuit 23, from a third working chamber 43, which is connected to the low-pressure side 27 of the accumulator circuit 23, and from a fourth working chamber 45 that is at atmospheric pressure. A pneumatically charged hydraulic accumulator 47, which is connected additionally on the low-pressure side 27, serves as volume compensator in the instance of unsteady events or due to temperature fluctuations. Moreover, a permanent fluid volume supply (not shown) is connected at a feed-in point 49, wherein said permanent fluid volume supply is connected via a feed-in line 51 and via a non-return valve 53 to the connection point 31 and thus to the low-pressure side 27 of the accumulator circuit 23. The feed-in line 51 leads from the connection point 31 via a pressure relief valve 55 to tank T. This constant pressure supply ensures that the low-pressure side 27 is maintained at the system-specific charging pressure level and that the overall system is filled via the feed-in line 51 if volume losses need to be replenished, for example if in the instance of pressure spikes fluid flows via the pressure relief valve 21 and/or 55 to tank T.

The valve control device 17 provides in the load lines 13, 15 a pair of first valve devices, each of which consists of a 2/2-way valve 57 and 58 respectively, each with a non-return function, which in their open position connect the two pump motor units 5 and 7 together, and which in their closed position uncouple the respective high-pressure side of the overall system from the low-pressure side, wherein, depending on the orientation of the non-return function, the displacement unit 1 is assigned to the high-pressure side and the displacement unit 3 is assigned to the low-pressure side of the overall system. Disposed between the load lines 13 and 15 and the low-pressure side 27 of the accumulator circuit 23 is a pair of second valve devices, each of which also consists of a 2/2-way valve 59 and 60 respectively, each with a non-return function. Moreover, disposed between the load lines 13 and 15 and the connection point 29 with the high-pressure side 25 of the accumulator circuit 23 is a pair of third valve devices with proportional and shut-off function, each of which consists of a 2/2-way valve 61 and 62 respectively.

Disposed in the line section of the load lines 13, 15 between the directional valves 57 and 58 respectively and the directional valves 61 or 62 respectively, connected in parallel to the latter, a pair of further, fourth valve devices, which comprise pressure relief valves 63 and 64 respectively, each with proportional pressure relief function. Disposed between the connection point 29 of the high-pressure side 25 of the accumulator circuit 23 and the connecting line 67 between the pressure reducing valves 63 and 64 is a further non-return valve 69. The overall system is completed by a pressure relief valve 71, located in the connecting line 73 that extends between the connecting points 29 and 31, that is, the connecting points of the accumulator 23.

The following FIGS. 2 to 6 depict the fluid paths for a number of different operating states yet to be described, wherein the fluid paths in connection with the high-pressure side 25 of the accumulator circuit 23 are shown in full lines, and the fluid paths in connection with the low-pressure side are shown in broken lines, and the flow direction is indicated by arrows.

The FIG. 2 depicts the operating state when charging the double-piston accumulator 33 where the traction drive is at standstill. The prerequisite for charging is a running combustion engine as well as free accumulator volume. If the traction drive is at standstill, the pump motor units 5 and 7 are in neutral position to start with, without displacement, hence no hydraulic fluid flows via these units. To provide a volume flow for the charging process, the pump motor unit 5 is pivoted and the proportional directional valve 62 is switched with maximum deflection, including valve 58. For the feeding process the pivoted pump motor unit 5 demands volume from the first load line 13, which flows from the third working chamber 43 on the low-pressure side of the double-piston accumulator 33 via the connection point 31 and flows via the non-activated directional valve 59 with open non-return function, as well as the opened directional valve 57, to the pump motor unit 5. The transported volume flows via the fully opened directional valve 62 to the working chamber 41 on the high-pressure side of the double-piston accumulator 33. In this instance the pressure relief valve 64 is set to at least the charging pressure in the second load line 15 to avoid a volume flow through it. If the maximum accumulator pressure is exceeded, the pressure relief valve 71 is activated and leads the charging volume flow away to the low-pressure side 27. From there it flows via the non-return function of the non-activated directional valve 59 to the first load line 13 and thus equalises the volume balance.

The FIG. 3 depicts the operating state when charging the double-piston accumulator 33 in drive mode. Again, the prerequisite for charging is a running combustion engine as well as free accumulator volume. In drive mode both displacement units 1 and 3 and the pump motor units 5 and 7 are pivoted and pressure medium circulates between the displacement units 1 and 3. To generate the necessary volume flow for the charging process, the pump motor unit 5 of the displacement unit 1 is pivoted further so that more volume can be fed into the second load line 15 than can be absorbed by the second displacement unit 3. This increases the pressure in the load line 15. As soon as the switch pressure set on the pressure relief valve 64 is reached, volume flows via the downstream non-return valve 69 to the high-pressure side working chamber 41 of the double-piston accumulator 33 and charges it. The volume displaced in the double-piston accumulator 33 on the low-pressure side working chamber 43 flows via the non-return function of the non-activated directional valve 59 to the first load line 13 where it combines with the volume that circulates in drive mode. As soon as the maximum accumulator pressure is exceeded, the pressure relief valve 71 switches and discharges a charging volume flow to the low-pressure side 27, from where it flows via the non-return valve function of the directional valve 59 to the first load line 13 and thus equalises the volume balance.

The FIG. 4 depicts the operating state “Brake Energy Recovery/Overspeed Protection”. The prerequisite for the recovery and accumulation of brake energy is an available accumulator volume in the double-piston accumulator 33 and a vehicle in motion (kinetic energy). The reason why the combustion engine must be protected from overload/overspeed is usually a force that acts on the vehicle, for example potential energy on a decline. In the instance of a traction drive in motion with the pressure medium in circulation, shown in the diagram of FIG. 4 anti-clockwise, the intention is to retard or maintain the driving speed and the engine speed. In drive mode both displacement units 1 and 3 are in operation, wherein the pump motor units 5 and 7 are fully or partially pivoted.

To regenerate brake energy or to provide overload/overspeed protection for the combustion engine, the pump motor unit 5 of the displacement unit 1 is pivoted back so as to reduce the torque that acts on the shaft 9 to such an extent that the load acting on the combustion engine is no longer able to cause any overspeed. As a result the pressure in the first load line 13 rises since the second displacement unit 3 delivers more than the displacement unit 1 can accept. Said rising pressure now generates a braking moment in the displacement unit 3, which slows down the vehicle. This may also be referred to as brake energy. In this instance the pressure relief valve 63 is actuated when the set switching pressure is reached, thus limiting the pressure in the load line 13. The volume discharged via the pressure relief valve 63 flows via the non-return valve 69 to the high-pressure side 25 of the double-piston accumulator 33 and charges the same. The volume displaced in the double-piston accumulator 33 flows from the low-pressure side 27 via the directional valve 60 to the load line 15. If the maximum accumulator pressure is exceeded, the pressure relief valve 71 is activated and discharges the charging volume flow towards the low-pressure side 27. To equalise the volume balance, volume is discharged from said low-pressure side 27 via the non-return valve function of the non-activated directional valve 60 to the load line 15. Moreover, the volume displaced from the double-piston accumulator 33 of the low-pressure side 27 flows via the directional valve 60 to the load line 15. The entire volume that comes together in the load line 15 now flows to the suction side of the pump motor unit 7 of the displacement unit 3, thus ensuring that the volume balance of the system remains constant.

The FIG. 5 depicts the operating state “Brake Energy Recovery/Overspeed Protection”. The prerequisite for the hydraulic starting of the combustion engine is the presence of accumulated energy in the double-piston accumulator 33. In this instance the combustion engine as well as the entire traction drive are shut down, and the pump motor units 5 and 7 of the displacement units 1, 3 are initially in neutral position. On initiating the starting sequence, the pump motor unit 5 of the displacement unit 1 is fully pivoted. The directional valves 61 and 60 are then activated, wherein the directional valve 57 may also be activated so as to avoid pressure spikes on the second displacement unit 3. The pressure medium now flows from the high-pressure side working chamber 41 of the double-piston accumulator 33 via the directional valve 61 to the load line 13 and further to the first displacement unit 1. The pressure relief valve 63 should be set to at least the accumulator pressure in the load line 13 to avoid a volume flowing through it. In the displacement unit 1, hydraulic energy is converted into mechanical energy and generates a starting torque on the shaft 9. The volume flowing via the displacement unit 1 flows towards the load line 15 and via the directional valve 58 and the directional valve 60 to the low-pressure side 27 of the double-piston accumulator 33, where it replaces the volume previously displaced to the high-pressure side 25.

The FIG. 6 depicts the operating state “Boost”. The prerequisite for hydraulically boosting the drive is accumulated hydraulic energy in the hydraulic accumulator. In this instance the traction drive is in motion, and the pressure medium between the displacement units 1 and 3 circulates in anticlockwise direction. A sudden load spike on the drive, for example due to a steep incline, causes a boosting of the drive, whilst the pump motor units 5 and 7 of both displacement units 1 and 3 are fully or partially pivoted. To facilitate the boosting process, the directional valve 57 is activated first and then the directional valves 61 and 59. The advanced activation of directional valve 57 ensures that the pressure medium can only flow in the intended direction via directional valve 57 despite the pressure drop. The directional valve 63 should be set to at least the working pressure present in the load line 13 to avoid a volume flow through it. As a result of activating the directional valves 61 and 59 the pressure medium now flows from the high-pressure side 25 of the double-piston accumulator 33 via the directional valve 61 to the load line 13 and further to the displacement unit 1. This raises the pressure level at the suction side of the pump motor unit 5 of the displacement unit 1. To provide the required pressure on the pressure side of the displacement unit 1 (in load line 15) only a small amount of mechanical power is therefore required from the combustion engine, which relieves the load on the combustion engine. The pressure medium flows from the displacement unit 1 via the load line 15 through the directional valve 58 to the second displacement unit 3, where the hydraulic energy is converted into mechanical energy and transmitted to the drive train. In this instance it is necessary that the pressure relief valve 64 is set to the maximum pressure in the load line 15 to avoid a volume flow through it. The pressure medium flows from the first displacement unit 1 via the load line 15 via the displacement unit 7 through the directional valve 59 and on to the low-pressure side 27 of the double-piston accumulator 33, where the volume previously displaced from the high-pressure side 25 is replaced.

The invention makes it possible to set the desired working pressure in the load line 13 proportionally. To this end the proportionally operating directional valve 61 is set to the desired pressure and opens up to a corresponding opening cross-section. The volume balance of the system remains the same in all described processes. Volume is simply shifted from one side of the double-piston accumulator 33 via the accumulator circuit 23 and the displacement units 1, 3 to the other side, through which energy is emitted or absorbed and accumulated. A closed traction drive system is realised in this manner. A volume flow passes mainly through the non-return valve 53, acting as shut-off valve, during the filling process of the system and caps the constant volume supply from the feed-in point 49 as soon as the low-pressure side 27 of the double-piston accumulator 33 as well as the two load lines 13 and 15 are at the set minimum system pressure.

However, no additional volume is required in the instance of a later “reallocation” of the double-piston accumulator 33 volume. Thus the pressure relief valve 55 remains shut. It serves simply to provide pressure protection to the low-pressure side 27 and can be omitted for the remaining consideration of the function. The same applies for the pressure relief valve 71, which limits the maximum pressure of the high-pressure side 25 of the double-piston accumulator 33.

Load spikes on the combustion engine can not only be generated by the traction drive but also by the functions of a hydraulic power circuit, the hydraulic pump of which is usually driven by the drive shaft 9 of the displacement unit 1. Nevertheless, even these load spikes that act on the combustion engine can be absorbed through the described boost process. The pair of directional valves 61 and 62 is preferably implemented in the associated valve block as two units that are functionally separated. For each unit a 2/2-way valve, double-sealed, may be provided for connecting the high-pressure side 25 to the load line 13 and to the displacement unit 1 during starting of the combustion engine, and a proportional pressure relief valve may be provided to be able to set the pressure of the load lines 13, 15. Moreover, instead of the proportional valve 62, a shut-off valve for charging the accumulator when at standstill may be used.

A combination of both functions proved to be rather problematic since a large cross-section has to be opened up (to achieve a low pressure drop) when starting the combustion engine, and where for the pressure control function a significantly smaller cross-section at high resolution (precision control edge) is required. For different possible designs of the displacement units 1 and 3 (such as constant, variable or mooring pumps) or for varying functions of the drive (e.g., boosting in forward and reverse mode without mooring pumps, or boosting in forward mode only) the functional units of the directional valves 61 and 62 may be provided with only one of the two required functions described. In the instance of relatively large flow rates it is desirable to provide all functional units of the valve control device 17 as pilot-controlled valves.

It should also be mentioned that the overall system is symmetrically identical in its structure, and that all described functions can also be performed in reverse order. Concerning the definition of the low-pressure and high-pressure side, such definitions depend on the respective operating state and can vary accordingly. The above-described valve devices 57 and 58 may also be viewed as non-return valves as far as their function is concerned. The drive solution according to the invention achieves a largely constant pressure level at the low-pressure side due to the influence of the double-piston accumulator and the additional compensation volume (capacity) of the diaphragm accumulator in the low-pressure side. Said constant pressure level also serves as reference pressure for a number of feedforward control circuits in the valve block. A further essential aspect of the invention is the proportionally adjustable and system-size independent pressure control of the suction-side load line in boost mode. Moreover, a proportionally adjustable and system-size independent pressure control of the load line in overrun mode (kinetic and/or potential energy) is achieved. 

1. A hydrostatic drive as an overall system, comprising two fluidic displacement units (1, 3), which can be adjusted at least in respect of the volumetric flow, and of which one is coupled to a power unit (9) and the other is coupled to an output drive (11) and which can be connected to each other in the manner of a closed fluidic circuit, connected to which is an accumulator circuit (23) that comprises at least one accumulator device (33), and which is subdivided into a low-pressure side (27) and a high-pressure side (25), and comprises a valve control device (17) for controlling the overall system.
 2. The hydrostatic drive according to claim 1, characterized in that, when the fluidic circuit is closed, both displacement units (1, 3) are connected to each other on their respective input and output side each with a load line (13, 15), wherein the valve control device (17) is disposed essentially between the two load lines (13, 15).
 3. The hydrostatic drive according to claim 1, characterized in that the valve control device (17) is provided with two first valve devices (57, 58) which, in their open position, connect the two displacement units (1, 3) via the respective load line (13, 15), and which in their shut-off position disconnect at least the high-pressure side from the low-pressure side of the overall system.
 4. The hydrostatic drive according to claim 1, characterized in that the displacement unit (3) of the output drive side is assigned to the low-pressure side and the displacement unit (1) of the power unit side is assigned to the high-pressure side of the overall system.
 5. The hydrostatic drive as an overall system, characterized in that a second valve device each (59, 60), with non-return function, is placed in the circuit between the two load lines (13, 15) and the low-pressure side accumulator circuit (27).
 6. The hydrostatic drive according to claim 1, characterized in that a further third valve device (61, 62) with proportional function is connected between the two load lines (13, 15) and the high-pressure side accumulator circuit (25).
 7. The hydrostatic drive according to claim 1, characterized in that a permanent fluid supply (49) is connected to the low-pressure accumulator circuit (27) between the second valve devices (59, 60) and the output drive side displacement unit (3).
 8. The hydrostatic drive according to claim 1, characterized in that the permanent fluid volume supply (49) is connected to a feed line (51) at its inlet and which vents into a tank (T) at the outlet side, wherein the feed line (51) is permanently connected to the low-pressure accumulator circuit (27) via a connection point (31), and where a non-return valve (53) is disposed into the feed line (51) between the connection point (31) and the inlet, and where a pressure relief valve (55) is disposed in said feed line (51) between the connection point (31) and the outlet.
 9. The hydrostatic drive according to claim 1, characterized in that two further, fourth valve devices (63, 64) with proportional pressure relief function are disposed between the two load lines (13, 15), in the line section between the two first valve devices (57, 58) as well as the two third valve devices (61, 62), extending parallel to the latter.
 10. The hydrostatic drive according to claim 1, characterized in that between the pairs of third (61, 62) and fourth valve devices (63, 64) with their associated connecting lines, a further connecting line is placed, into which a further non-return valve (69) is inserted.
 11. The hydrostatic drive according to claim 1, characterized in that the accumulator device of the accumulator circuit (23) consist essentially of a double-piston accumulator (33), the double-piston (35) of which is guided longitudinally moveable in an accumulator housing (37), which separates a first working chamber (39), applied with a charging pressure, from a second working chamber (41) on the high-pressure side (25) as well as from a third working chamber (43) on the low-pressure side (27) and a fourth working chamber (45) at atmospheric pressure from each other.
 12. The hydrostatic drive according to claim 1, characterized in that the respective displacement unit (1, 3) consists of a four-quadrant system with adjustable pivoting angle, which may be operated as a hydraulic motor as well as a hydraulic pump.
 13. A valve control device as a subsystem, in particular for an overall system in form of a hydrostatic drive according to one of the preceding claims, comprised at least of one valve pair each of a first valve device (57, 58) for the selective opening or closing of two control lines (13, 15) between two displacement units (1, 3); a second valve device (59, 60) with a non-return function; a third valve device (61, 62) with proportional function; and a fourth valve device (63, 64) with proportional pressure relief function.
 14. The valve control device according to claim 13, characterised in that a pressure relief valve (71) is disposed in the accumulator circuit (23) between a connection point (31) on a permanent fluid volume supply (49) and a further connection point (29) between the pair of third valve devices (61, 62). 